A. C.
Baptista
a,
I.
Ropio
a,
B.
Romba
a,
J. P.
Nobre
a,
C.
Henriques
b,
J. C.
Silva
b,
J. I.
Martins
cd,
J. P.
Borges
*a and
I.
Ferreira
*a
aCENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. E-mail: jpb@fct.unl.pt; imf@fct.unl.pt
bCENIMAT/I3N, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
cUniversidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Química, 4200-465 Porto, Portugal
dLab2PT, Instituto de Ciências Sociais, Universidade do Minho, 4710 - 057 Braga, Portugal
First published on 30th November 2017
A novel cellulose-based bio-battery made of electrospun fibers activated by biological fluids has been developed. This work reports a new concept for a fully organic bio-battery that takes advantage of the high surface to volume ratio achieved by an electrospun matrix composed of sub-micrometric fibers that acts simultaneously as the separator and the support of the electrodes. Polymer composites of polypyrrole (PPy) and polyaniline (PANI) with cellulose acetate (CA) electrospun matrix were produced by in situ chemical oxidation of pyrrole and aniline on the CA fibers. The structure (CA/PPy|CA|CA/PANI) generated a power density of 1.7 mW g−1 in the presence of simulated biological fluids, which is a new and significant contribution to the domain of medical batteries and fully organic devices for biomedical applications.
A. C. Baptista5 and co-workers proposed a new concept of a flexible and lightweight cellulose-based battery, so called bio-battery, activated by biological fluids. For concept demonstration, Al and Ag metallic thin films were deposited on each side of an ultrathin cellulose-based electrospun membrane as anode and cathode, respectively. This device, with an area of 2 cm2 and a thickness of 53 μm, placed in contact with simulated body fluids (<0.1 ml) displayed a power density of 3.38 μW cm−2. Later, Yong Kong9 and colleagues proposed a bio-battery composed of PPy doped with a biological polyelectrolyte (dextran sulfate, DS) as cathode and a bioresorbable Mg alloy as anode. This battery exhibited an energy density of 790 W h kg−1, using commonly biological media as electrolyte. Recently, Sha Li6 used cellulose-based composites as cathodes for zinc–air bio-batteries activated by simulated body fluids. PPy/CNTs composites were chemically synthetized and deposited on cellulose filter paper for the cathode while a zinc foil was used as anode. In the presence of simulated biological fluids, this device discharged in 24.5 hours at a current density of 60 μA cm−2. X. Jia8 and co-workers reported the development of a magnesium–air bio-battery using as cathode a silk fibroin–polypyrrole (SF–PPy) film, a bioresorbable Mg alloy as anode and a phosphate buffered saline (PBS) solution as electrolyte. This bio-battery exhibited a discharge capacity up to 3.79 mA h cm−2 for a current of 10 μA cm−2 at room temperature.
Electrically conductive polymers, such as PPy and PANI, are biocompatible and are therefore valuable materials that can be used as electrodes in lightweight and flexible biomedical batteries. The ability to customize conductive nanostructures to meet the requirements of specific applications gives electrospinning an advantage over other production methods. However, the electrospinning of continuous conductive fibers directly from conductive polymeric solutions is a great challenge.10 Some authors have proposed the addition of a carrier polymer to facilitate the electrospinning process; others reported on the combination of the electrospinning technique with a nanocoating procedure.11–13 The deposition of PPy or PANI on the surface of fabrics and yarns has been widely investigated in the last few years in order to incorporate these polymers in new and functional devices, such as sensors,14 biosensors15 and scaffolds for tissue engineering.16
Considering the growing need for power source miniaturization and the replacement, cost and risk inherent to conventional implantable medical devices, there is a need for the development of new electrical power source concepts. This work presents the development of flexible, lightweight non-toxic and conductive cellulose-based electrospun fibers functionalized with PPy and PANI. In order to obtain highly conductive fibers, the in situ polymerization of Py and Ani was carried out on the surface of cellulose acetate electrospun fibers. The polymerization conditions were extensively studied and lately the composite membranes were evaluated as electrodes for bio-batteries.
A fully polymeric bio-battery was constructed by assembling the CA/PPy and CA/PANI composite membranes separated by a CA electrospun membrane and tested with physiological simulated solution as electrolyte. The fully organic device reported here gives a new contribution towards the state of the art of bio-batteries taking advantage of the ionic content present in biological fluids, such as blood and sweat to supply low power consumption medical devices.
Fig. 1 Illustration of the methodologies followed for the preparation of CA fibers coated with (a) PPy and (b) PANI by in situ chemical polymerization. |
The dependence of electrical conductivity on the Ox/Mon ratio of CA/PPy fibers obtained after 1 hour of polymerization using a pyrrole concentration of 0.025 M is shown in Fig. 2a. The electrical conductivity increased reaching maximum values around 5 × 10−3 S cm−1 as FeCl3/Py increased up to 2. Considering that the FeCl3 amount should be as small as possible due to its toxicity, the Ox/Mon ratio of 2 was chosen to carry out Py polymerization. Since cellulose can degrade under prolonged polymerization reaction or under aggressive reaction conditions,17 the influence of monomer concentration and polymerization time on the electrical conductivity was investigated (Fig. 2b). The conductivity increases remarkably up to values of 10−2 S cm−1 at 15 and 30 minutes of polymerization, for monomer concentrations of 0.075 and 0.05 M, respectively. This behavior is attributed to the high PPy yield and the formation of a continuous layer on CA nanofibers surface. However, for lower monomer concentrations, more than 90 min are required to achieve a conductivity close to 10−2 S cm−1. The morphology of CA/PPy composite fibers obtained using pyrrole concentrations of 0.025 M (Fig. 2c), 0.05 M (Fig. 2d) and 0.075 M (Fig. 2e) and a polymerization time of 45 min was observed by SEM. The PPy coating formed on CA fibers is not continuous for the lower monomer concentration 0.025 M, which explains the electrical conductivity measured. A continuous coverage of fibers and optimized conductivity values were obtained for monomer concentration of 0.05 M and a polymerization time of 30 min. PPy tends to form aggregates when the monomer concentration increases to 0.075 M, which explains the slight decrease in electrical properties. Some aggregates are also evident for the monomer concentration of 0.05 M but for a longer polymerization time (1440 min) – Fig. 2f. Both electrical and morphological results show that the most adequate conditions to obtain PPy-coated CA fibers with high electrical conductivity are the following: Py at a concentration of 0.05 M, an Ox/Mon ratio of 2 and a reaction time of 30 min.
The TEM images of PPy coated CA nanofibers in the optimized synthesis conditions are shown in Fig. 2g and h. Nonwoven composite fibers have an average diameter of 290 ± 69 nm and a uniform and continuous PPy coating with a thickness in the range of 40–50 nm.
The electrical conductivity of polyaniline can change from 10−9 to 100 S cm−1 depending on its oxidation state, degree of protonation and type of dopant used. A higher conductivity is obtained for stronger protonic acids, such as HCl, as dopant agent. In this work, the synthesis of PANI using ammonium persulfate ((NH4)2S2O8), as the oxidant agent was investigated in detail to find the synthesis conditions that would lead to highly conductive fibers. Reports found in the literature have explained the nano-assembly of PANI on a cellulose-based network as a result of the interaction between the protonated nitrogen of PANI (in acidic medium) with the hydroxyl groups of cellulose through hydrogen bonds.18
The influence of monomer concentration, Ox/Mon molar ratio and reaction time on the electrical conductivity of CA/PANI fibers is shown in Fig. 3a. The pristine CA electrospun fibers have an electrical conductivity of (7.1 ± 0.8) × 10−11 S cm−1. During aniline polymerization, PANI gradually grows onto CA fibers forming a continuous sheath and the conductivity increases to values above 10−3 S cm−1 for a polymerization period of 30 min. Further increase in conductivity of the membranes up to 1 S cm−1 was achieved by adjusting polymerization time and Ox/Mon ratio.
The granular aspect of the PANI formed on the fibers along the polymerization time is seen in Fig. 3b–d, for an Ox/Mon molar ratio of 0.5. Fig. 3b clearly shows a non-uniform coating for 30 min of polymerization and an increase of PANI aggregates on the fiber's surface with the polymerization time increase to 45 min and 60 min (Fig. 3c and d, respectively). For these polymerization periods the original structure of CA membrane, composed of individual cylindrical fibers, is not preserved due to aggregation of PANI particles forming a bulk-like material. Since the agglomeration of PANI particles took place on the surface of CA fibers for a monomer concentration of 4 M, a lower concentration, 2 M, was studied for two Ox/Mon ratios (0.25 and 0.5). A decrease of conductivity is observed in Fig. 3e for the Ox/Mon ratio of 0.25, indicating that the amount of monomer used was not enough to yield fibers completely covered by PANI even for prolonged polymerization times. Considering an Ox/Mon ratio of 0.5, the composites produced showed electrical conductivities of 10−1 S cm−1 for 45 and 60 min of polymerization. SEM images (Fig. 3f and g) confirmed the existence of complete fiber coverage by PANI aggregates on the surface of CA fibers.
Overall, we concluded that a continuous and uniform PANI coating of CA fibers with high electrical conductivity can be obtained with aniline at a concentration of 2 M, an Ox/Mon ratio of 0.5 and a 30–45 min reaction time. CA/PANI composite fibers produced under these conditions present an average diameter of 577 ± 59 nm while suggesting the formation of a PANI layer in the range of 160–170 nm.
The CA nonwoven membrane is composed of nanofibers that are loosely packed together without any chemical crosslinking point between fibers. Consequently, its tensile strength is relatively low. According to Fig. 4a, chemical polymerization of Py and Ani on the surface of CA electrospun fibers did not significantly affect the membranes' mechanical properties. The exception is the slight increase of the Young's modulus observed for CA/PPy membranes (42 ± 14 MPa for CA and 59 ± 13 MPa for CA/PPy) that may be due to the nanosized PPy particles on the surface of fibers increasing the cohesion of the membrane at the crossing points between fibers.
Although some authors have suggested that PPy and PANI can generally be regarded as a biocompatible synthetic polymer, it is very important to understand if the processing and morphology of the composites has affected the toxicity of the material.19,20 Following standard cytotoxicity test methods (ISO-10993-5), cells were exposed to extracts obtained by placing the CA/PPy and CA/PANI fibrous membranes in cell culture media. Cell viability was used as an indicator of toxicity and assessed using the resazurin reduction test – Fig. 4b. Both relative cell populations are above 90% indicating that the materials tested are free of harmful extractables or these are at a too low concentration to cause any acute cytotoxic effects capable of affecting cell proliferation and enzymatic activity.
To evaluate the composite materials described above as an electrode material for energy harvesting, CA/PPy and CA/PANI were assembled in the construction of a bio-battery. The CA/PPy and CA/PANI composite membranes were used as electrodes, separated by a cellulose acetate electrospun membrane that acts as the separator. The CA/PPy is the negative electrode and the CA/PANI is the positive electrode where the oxidation of PPy and the reduction of PANI occur, respectively, according to reactions (1) and (2) for discharge. Eventually, the O2 can also be reduced in the CA/PANI electrode configuring the system's hybrid operation.
The electrochemical behavior of the CA/PPy|CA|CA/PANI structure was evaluated by cyclic voltammetry in the presence of a physiological solution – 0.9% (w/v) NaCl solution.
The voltammogram obtained for CA/PPy as the working electrode shows that the anodic wave begins at −0.2 V with a shoulder around 0.08 V and a peak at 0.23 V, and the cathodic sweep shows two reduction peaks at −0.14 V and −0.25 V (Fig. 5a). The redox couples are associated with the oxidation of PPy to polaron and bipolaron states,21 as shown in Fig. 5b. Previous studies have reported that when potentials higher than 0.9–1.0 V vs. SCE are applied, PPy electrodes lose their electrochemical activity due to an irreversible redox reaction.22,23 This phenomenon was not observed during the electrochemical characterization. When CA/PANI is connected as the working electrode, the voltammograms show two redox pairs, with oxidation peaks at 0.23 V and 0.65 V, and reduction peaks at −0.25 V and −0.60 V (Fig. 5c). The first redox couple indicates the oxidation of leucoemeraldine form to protonated emeraldine, and the second couple to oxidation of emeraldine form to pernigraniline, as shown in Fig. 5d. The oxidation of emeraldine is generally considered to be less reversible than the well-known emeraldine–leucoemeraldine transition, and the oxidation products are easily hydrolyzed in acidified aqueous medium without aniline leading to the progressive degradation of the polymer.24–27 Therefore, the current density of the peaks decreases with the number of scanning cycles.
The redox reactions occurring on the individual electrodes during charging under the prevailing experimental conditions can be summarized as follows (while the reverse reactions prevail for discharging):
Positive electrode
[–B–NH–B–NH+QNH+–B–NH–]2x → [–B–NQN–]4x + (8x)H+ + (4x)e− | (1) |
Negative electrode
[–C4H3Ny+–]x + (xy)e− → [–C4H3N–]x | (2) |
The maximum power density (Pmax) of 1.7 mW g−1 (0.8 mW cm−3) was determined from I–V curves obtained from cyclic voltammetry (Fig. 5e). The power density of the devise matches the values found for typical pacemakers that require less than 10 μW to operate.
Short charge/discharge measurements (one cycle of 4 min) were carried out at different charge/discharge rates using a succession of decreasing charge/discharge current densities. Fig. S1 (in ESI section†) shows a decrease in ohmic losses with current density decreasing. The small discrepancy of the final charge potential for the same current density can be attributed to small current oscillations and/or structural modifications of polymers.
The device was submitted to charge/discharge cycles at a constant charge/discharge current density of 5.3 mA h g−1 for approximately 5 hours. The amount of electrolyte used was 20 μl, before measurements starts (1st cycle for t = 0 s), and no further addition has been performed during the following cycles. Fig. 5f shows the coulombic efficiency determined for the 5 cycles. The decrease of efficiency in 20% after the 5th cycle can be explained by the need of electrolyte addition. Since the device is not sealed, the small amount of electrolyte added when the measurement starts can be insufficient to guarantee the electrochemical stability of the charge/discharge experiments during prolong periods of time.
This hybrid device differs from conventional batteries that comprise alkali-metals-containing intercalation or insertion materials as electrodes in a closed system, since its concept beyond the redox characteristics of conductive polymers electrodes is also related with the active material of the circulating physiological fluid.
In addition to being flexible, lightweight and ultra-thin (<300 μm of thickness), this fully polymeric bio-battery has the advantage of having an economical production process making it a promising alternative to power implantable and portable microwatt electronic devices. Comparing the main properties of the proposed device with other ultralow power sources found in the literature (Table 1), we conclude that its power density per volume is competitive. The power density is 8 times higher than the first concept5 reported in 2011 using novel and non-toxic electrodes materials.
In summary, conductive CA/PPy and CA/PANI composite fibers have been obtained with the advantage of preserving the main properties of electrospun membranes, such as the flexibility, porosity and large surface area, making them suitable electrodes for the bio-battery. A fully polymeric bio-battery was developed and validated demonstrating promising performance results, thus making it a new, economic alternative for supplying low-power consumption medical devices.
σ = l/AR | (3) |
Ten samples of each membrane (CA, CA/PPy and CA/PANI) were analyzed to obtain mean values for the Young's modulus (E), the ultimate tensile strength (TS) and ultimate tensile strain (ε). The samples were kept in a desiccator under controlled humidity (40–45%) and room temperature (∼25 °C) before the stress–strain testing. The applied deformation rate was 1 mm min−1 and the temperature was kept constant at 25 °C.
The cell was constructed using two identical pieces of the conductive materials (CA/PPy as the anode and CA/PANI as the cathode) separated by a CA electrospun membrane. The active materials have a total mass of 1.9 mg, approximately.
The setup (Fig. S2 – ESI†) was filled with 20 μl of electrolyte solution (0.9% NaCl) and no electrolyte refresh has been made during the measurement. It is important to note that the cell is not sealed.
Different charge/discharge rates were evaluated. For such analysis, short-time charge/discharge measurement (one cycle of 4 min) were tested at charge/discharge current densities of 20 mA g−1, 10.5 mA g−1, 5.3 mA g−1, 2.3 mA g−1, 1.3 mA g−1 and 0.7 mA g−1.
A cycling test of approximately 5 hours was performed by repeatedly charging (for 30 min) and discharging (also for 30 min) the device at a constant current density of 5.3 mA g−1. A lower cut-off potential of 0 V and an upper cut-off potential of 0.8 V was used for all experiments.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta06457h |
This journal is © The Royal Society of Chemistry 2018 |